1. Field of the Invention
The present invention relates to a sensor device using a terahertz wave containing electromagnetic wave components in a frequency domain from a millimeter wave band to a terahertz wave band (not less than 30 GHz and not more than 30 THz), and a sensing system or an imaging system using the same.
2. Description of the Related Art
Recently, non-destructive sensing technology using terahertz waves (THz waves) has been developed. As an application field of electromagnetic waves in this frequency band, there is technology for safe fluoroscopic inspection equipment as an alternative to X-ray equipment to perform imaging. Further, there have been developed spectroscopic technology for determining the absorption spectrum or complex permittivity inside a substance to examine physical properties such as the bonding state of molecules, measurement technology for examining physical properties, such as carrier concentration or mobility, and electric conductivity, and biomolecule analysis technology. As the method of generating a terahertz wave, a method using nonlinear optical crystal is widely used. Typical nonlinear optical crystals include LiNbOx (hereinafter, also referred to as LN), LiTaOx, NbTaOx, KTP, DAST, ZnTe, GaSe, GaP, and CdTe. Second-order nonlinear phenomena are used for generation of a terahertz wave. As a system, there is known difference-frequency generation (DFG) using incidence of two laser beams having a frequency difference. In the DFG system, when two laser beams different in frequency are entered, nonlinear polarization having a cycle corresponding to a difference frequency of the two laser beams occurs. In the nonlinear optical crystal, the energy state is excited by the incidence of the laser beams and an energy wave is radiated when returning to the original energy state. When the nonlinear optical crystal is nonlinearly polarized, an energy wave corresponding to the polarized frequency is radiated, while when it is polarized with a frequency of a terahertz wave, the terahertz wave is radiated from the nonlinear optical crystal. There are also known a system for generating a monochromatic terahertz wave by an optical parametric process, and a system for generating a terahertz wave pulse by optical rectification with radiation of a femtosecond pulsed laser beam.
As a process of generating a terahertz wave from such nonlinear optical crystal, electro-optic Cerenkov radiation has recently drawn attention. This is a phenomenon in which, as illustrated in FIG. 8, a terahertz wave 101 is radiated in a conical shape like a shock wave when a group velocity of propagation of a laser beam 100 as an excitation source is faster than a propagation phase velocity of the generated terahertz wave. A radiation angle θc (also called “Cherenkov angle”) of the terahertz wave is determined by the following equation according to a ratio of refractive indexes in a medium (nonlinear optical crystal) between light and the terahertz wave:cos θc=vTHz/vg=ng/nTHz  (1).
where vg and ng denote the group velocity and group refractive index, respectively, and vTHz and nTHz denote the phase velocity and refractive index of the terahertz wave, respectively. Up to now, there has been reported that a high-intensity terahertz pulse is generated by optical rectification using the Cerenkov radiation phenomenon by entering a femtosecond laser beam with inclined wavefront into LN (J. Opt. Soc. Am. B, vol. 25, pp. B6-B19, 2008). Further, it is described that a monochromatic terahertz wave is generated by a DFG system using a slab waveguide having a thickness sufficiently smaller than the wavelength of the generated terahertz wave to eliminate the necessity of wavefront tilt (Japanese Patent Application Laid-Open No. 2010-204488 (Patent Document 1)).
The examples described in the aforementioned conventional art documents are related to a proposal of performing phase matching in the radiation direction between terahertz waves generated from different wave sources because the terahertz waves are generated by traveling-wave excitation to reinforce the terahertz waves with each other in order to improve extraction efficiency. A terahertz wave generated from a slab waveguide propagates in an adjacent coupler (Si prism in the case of Patent Document 1) and is extracted from the coupler into a space. The features of this radiation system include the fact that a high-intensity terahertz wave can be generated with relatively high efficiency among those using nonlinear optical crystal, and the fact that the terahertz wave band can be widened by selecting absorption in a terahertz region due to a phonon resonance unique to the crystal on a high frequency side. Compared with terahertz generation by a photoconductive device, these techniques can widen the generation band, and in the case of generating a terahertz wave pulse with optical rectification, the pulse width can be reduced. Therefore, it is expected that the device performance can be improved when the device is used in a terahertz time-domain spectroscopic apparatus, for example.
However, in the systems described in the aforementioned conventional art documents, Cerenkov radiation of a terahertz wave generated in nonlinear optical crystal (which is the term used in these documents, and in this specification, the term “electro-optic crystal” as an approximately equivalent term is used) and propagating in the coupler is all extracted into a space. Then, light is focused on a sample desired to be sensed as necessary by means of a parabolic mirror or a lens to analyze a microscopic region of the sample. In this case, since the wavelength of a terahertz wave used is typically about a few hundred μm, light can only be condensed up to a beam diameter corresponding to the wavelength due to the diffraction limitation. The reality is that the spatial resolution is generally in millimeters though it depends on the optical system. This makes it difficult to sense a microscopic sample or to deal with imaging of a component distribution at a resolution equal to or less than the wavelength. To respond to a request for observation at an improved spatial resolution, it is necessary to use known near field technology in an optical region.